Magnetic resonance thermometry method

ABSTRACT

A method for reducing errors in the measurement of temperature by magnetic resonance, for use in magnetic resonance imaging-guided HIFU equipment, includes acquiring an MR phase image, as a reference image, before heating an area to be heated with the HIFU equipment; acquiring another MR phase image, as a heated image, during or after the heating by the HIFU equipment; and calculating the temperature change in the heated area according to said heated image and said reference image; and making compensation to said temperature change according to the change in the magnetic field caused by the position change of an ultrasonic transducer in said HIFU equipment. The method can reduce significantly the temperature errors resulting from the position changes of the ultrasonic transducer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of high intensity focusedultrasound (HIFU) monitored by magnetic resonance imaging (MRI) and,particularly, to a method for reducing errors in the measurement oftemperature of MRI guided HIFU equipment.

2. Description of the Prior Art

Magnetic resonance (MR) thermometry based on proton resonance frequency(PRF) shift can be used to monitor temperature changes in an area heatedby HIFU in MRI-guided HIFU equipment, based on the phenomenon of theresonance frequency of the protons in water being offset (shifted)dependent on the temperature change. MR thermometry based on PRF-shiftrequires that a base image (MR phase image) before heating, alsoreferred to as a reference image, be generated, with the reference imageproviding information on a reference phase. By subtraction from thephase image (also referred to as a heated image) acquired during heatingor after heating, the exact value of the elevated temperature in theheated area can be determined.

During a practical heating process, however, after the reference imageis acquired changes may occur in the position of the ultrasonictransducer (i.e. the treatment head), and the susceptibility changeresulting from the movements of the ultrasonic transducer causes changesin the static magnetic field of the focal region of the ultrasonictransducer, so that the subtraction of the heated image and thereference image produces an additional phase difference, thus causingerrors in the temperature measurement.

Currently, there are mainly two common solutions for reducingtemperature errors. One of the solutions can be referred to as a singlereference image method wherein after the reference image has beenacquired, the movement range of the ultrasonic transducer is restricted,so as to restrict the temperature errors within an acceptable range.However, since the spatial range used by a reference image is verysmall, while the ultrasonic transducer moves within a relatively largespatial range in the HIFU treatment process, it is necessary to acquirereference images frequently for various positions in order to measurethe temperature of each focal position of the ultrasonic transducer, andthis increases the complexity of the temperature measurement and theoverall treatment time.

The other solution for reducing temperature errors can be referred to asa self-reference method, i.e. not acquiring any reference images, butinstead utilizing the heated images themselves to obtain the referencephase by a polynomial fitting and extrapolation of the phase from thenon-heated region. The temperature change monitored using this method islimited to the vicinity of the focus of HIFU, and it is very difficultin practical applications to monitor the temperature changes outside thefocus point. Furthermore, the accuracy of the polynomial fitting andextrapolation and the complexity of the phase image are dependent on thesize of the heated area, and it is relatively difficult to obtainstable, consistent and accurate results in general.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method for reducingerrors in the measurement of temperature by a magnetic resonance imagingmethod, for use in obtaining accurate temperature change in a heatedarea.

The present invention provides a method for reducing errors in themeasurement of temperature by a magnetic resonance imaging method, foruse in MRI guided HIFU equipment, that includes acquiring an MR phaseimage, as a reference image, before heating an area to be heated withthe HIFU equipment; acquiring another MR phase image, as a heated image,during or after the heating by the HIFU equipment, and calculating thetemperature change in the heated area according to the heated image andthe reference image, and automatically compensating the temperaturechange according to a change in the magnetic field caused by a positionchange of an ultrasonic transducer in said HIFU equipment.

Preferably, the compensation of the temperature change is made accordingto the following equation,

${\Delta\; T} = {{\Delta\; T_{conv}} - \frac{y \cdot \lbrack {{\Delta\; B_{t{({r\; 2})}}} - {\Delta\; B_{t{({r\; 1})}}}} \rbrack \cdot T_{E}}{y \cdot B_{0} \cdot \alpha \cdot T_{E}}}$wherein, ΔT represents the value of the temperature change after beingcompensated; ΔT_(conv) represents the value of the temperature changecalculated according to the heated image and said reference image;[ΔB_(t(r2))−ΔB_(t(r1))] represents the change in the magnetic fieldcaused by the position change of the ultrasonic transducer from aposition r1 to position r2; γ represents the gyromagnetic ratio ofhydrogen atomic nuclei; B₀ represents the static magnetic fieldstrength; and α represents a temperature-frequency coefficient.

In a preferred embodiment, the method further includes measuring themagnetic field produced by the ultrasonic transducer in water as thechange in the magnetic field caused by the position change of theultrasonic transducer.

The ultrasonic transducer has a support (mount). During the measuring ofthe magnetic field caused by the ultrasonic transducer, the support isput into and taken out of water together with the ultrasonic transducer.

Preferably, the equationΔB _(t)=(φ₁−φ₂)/(γ·T _(E))is used to calculate the magnetic field ΔB_(t) caused by the ultrasonictransducer, wherein φ₁ represents a first phase image acquired when theultrasonic transducer is in water; φ₂ represents a second phase imageacquired when the ultrasonic transducer is not in water; γ representsthe gyromagnetic ratio of hydrogen atomic nuclei; and T_(E) representsan echo time.

In another preferred embodiment, the method further includes calculatingthe magnetic field caused by the ultrasonic transducer as the change ofthe magnetic field caused by the position change of the ultrasonictransducer.

The steps of calculating the magnetic field caused by the ultrasonictransducer include dividing the ultrasonic transducer into a number offinite volume elements, calculating the magnetic dipole moment of eachfinite volume element, and calculating the magnetic field in spaceproduced by the magnetic dipole moment of each finite volume element,and summing the magnetic field produced by each finite volume element toobtain the magnetic field caused by said ultrasonic transducer.

From the abovementioned solutions it can be seen that since the presentinvention compensates the temperature measurement according to themagnetic field caused by the ultrasonic transducer, it can reducesignificantly the temperature errors resulting from the position changesof the ultrasonic transducer. In comparison with the existing singlereference image method, the present invention does not need to acquire alarge number of reference images for different positions of theultrasonic transducer, so the complexity of the temperature measurementis reduced and the speed of the whole treatment process is increased. Incomparison with the existing self-reference method, the presentinvention can accurately obtain the temperature changes in the heatedarea, thus providing stable results of temperature measurement.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates the coordinate system of the ultrasonictransducer and the coordinate system of the magnet in an embodiment ofthe present invention.

FIG. 2 schematically illustrates the movement of the ultrasonictransducer in an embodiment of the present invention.

FIG. 3 schematically illustrates a device for measuring the magneticfield caused by the ultrasonic transducer, in accordance with thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a schematic diagram of a coordinate system of the ultrasonictransducer and a coordinate system of the magnet in an embodiment of thepresent invention, in which the coordinate system xyz represents thecoordinate system of the magnetic body, and the coordinate system x′ y′z′ represents the coordinate system of the ultrasonic transducer,wherein (a) represents that the ultrasonic transducer is located at theoriginal position, and (b) represents the position of the ultrasonictransducer after having been rotated about the direction of the magneticfield B₀.

FIG. 2 is a schematic diagram of the movement of the position of theultrasonic transducer in the embodiment of the present invention, inwhich 100 represents a water tank, 110 and 110′ respectively representthe ultrasonic transducer before and after being moved, 120 and 120′respectively represent the focus points corresponding to 110 and 110′,130 represents the body of a patient, and 140 represents the area heatedby HIFU equipment, such as a tumor.

FIG. 3 is a schematic diagram showing the device for measuring themagnetic field caused by an ultrasonic transducer, in which, 200represents a water tank, 210 represents the ultrasonic transducer, and220 represents the focus point of the ultrasonic transducer, and thesupport of the ultrasonic transducer is not shown.

In order to explain the objects, technical solutions and advantages ofthe present invention, the present invention will be described in detailbelow in the context of a particular embodiment.

Gradient echo sequences are used to measure an MR phase image, and sincethe local temperature in the measured tissue is changing, the protonresonance frequency changes with it, while the change of the protonresonance frequency can be reflected in the MR phase image. Accordingly,the temperature change can be expressed as:

$\begin{matrix}{{\Delta\; T} = \frac{\Delta\varphi}{\gamma \cdot B_{0} \cdot \alpha \cdot T_{E}}} & (1)\end{matrix}$in which, ΔT represents the temperature change, γ represents thegyromagnetic ratio of the hydrogen atomic nuclei (for a proton, itrepresents 42.58×10⁶ Hz/T), B₀ represents the static magnetic fieldintensity, T_(E) represents the echo time, α represents the temperaturefrequency coefficient, and Δφ represents the phase difference before andafter the ultrasonic transducer of the HIFU equipment releasingultrasonic energy (heating), that is:Δφ=φ_(T)−φ_(R)  (2)and in equation (2), the phase image φ_(R) is acquired before theheating by the HIFU equipment, and the phase image φ_(T) is acquiredwhen it is being heated by the HIFU equipment or after it has beenheated by the HIFU equipment.

The ideal magnetic field for the MRI equipment is a uniform field,however, since an inherent non-uniform field distribution ΔB_(c) existsin the actual magnetic field B, the actual magnetic field B is:B(x,y,z)=B ₀ +ΔB _(C)(x,y,z)  (3)

In the MRI guided HIFU equipment, the ultrasonic transducer will alsointroduce an additional magnetic field ΔB_(t) into the existing magneticfield. Since there is no nonlinear magnetic substance (ferromaterial,etc.) in the MRI imaging area, in the case of the ultrasonic transducertransversely moving or rotating about the B₀ direction, the spatialdistribution of ΔB_(t) is constant with respect to the ultrasonictransducer. If ΔB_(t) (x, y, z) is used to represent the inducedmagnetic field when the ultrasonic transducer is in the position r=(a,b, c), and ΔB_(t) (x′, y′, z′) is used to represent the induced magneticfield in the case of the ultrasonic transducer only transversely movingwithout rotating, ΔB_(t(r))(x, y, z) is a translation of ΔB_(t) (x′, y′,z′), which can be calculated by the following equation:ΔB _(t(r))(x,y,z)=ΔB _(t)(x−a,y−b,z−c)  (4)

As shown in FIG. 1, taking into consideration that the ultrasonictransducer is rotated at an angle θ along B₀, then ΔB_(t(r)) (x, y, z)can be calculated by the following equation:ΔB _(t(r))(x,y,z)=ΔB _(t)(x′,y′,z′)  (5)wherein

x′=x cos θ+y sin θ−a

wherein y′=−x sin θ=y cos θ−b

z′=z−c.

As shown in FIG. 2, when the ultrasonic transducer is located at theposition 1 (i.e. the position where the ultrasonic transducer 110 islocated), r1=(x₁, y₁, z₁), and this time, the magnetic field can beexpressed as:B _(R)(x,y,z)=B ₀ +ΔB _(c)(x,y,z)+ΔB _(t(r1))(x,y,z)  (6).

An MR phase image is acquired at the position 1 as a reference image,and this time, the measured phase image can be expressed as:φ_(R) =g·B _(R1) ·T _(E)  (7).

When the ultrasonic transducer is moved to the position 2 (i.e. theposition where the ultrasonic transducer 110′ is located), r2=(x₂, y₂,z₂), and this time, the magnetic field can be expressed as:B _(r) ₂ (x,y,z)=B ₀+Δ_(c)(x,y,z)+ΔB _(t(r) ₂ ₎(x,y,z)  (8).

An MR phase image is acquired at the position 2, and this time, theacquired phase image can be expressed as:φ_(T)=γ·(B _(r) ₂ ·α·ΔT _(E)  (9).wherein, ΔT is a value of temperature change in the heated area.

According to the equations (6) to (9):

$\begin{matrix}{{\Delta\; T} = {\frac{( {\varphi_{T} - \varphi_{R}} )}{\gamma \cdot B_{r_{2}} \cdot \alpha \cdot T_{E}} - \frac{\gamma \cdot \lbrack {{\Delta\; B_{t{({r\; 2})}}} - {\Delta\; B_{t{({r\; 1})}}}} \rbrack \cdot T_{E}}{\gamma \cdot B_{r\; 2} \cdot \alpha \cdot T_{E}}}} & (10)\end{matrix}$

In the equation (10), the first term is a value ΔT_(conv) of temperaturechange obtained by calculating the difference of the phase images, whichis equivalent to a value of temperature change obtained by theconventional PRF thermometry. A second term is a temperature errorcaused by the magnetic field change ΔB_(pos)(=ΔB_(t(r2))−ΔB_(t(r1))resulting from the position change of the ultrasonic transducer.

In practical use, since ΔB_(c) is at the level of only severalmillionths of the magnitude of the B₀, and the influence of ΔB_(t) on B₀can be ignored, B_(r2) in the equation (10) may be replaced by B₀.Therefore, the equation (10) can be converted into:

$\begin{matrix}{{\Delta\; T} = {{\Delta\; T_{conv}} - \frac{\gamma \cdot \lbrack {{\Delta\; B_{t{({r\; 2})}}} - {\Delta\; B_{t{({r\; 1})}}}} \rbrack \cdot T_{E}}{{\gamma \cdot B_{0}}{\alpha \cdot T_{E}}}}} & {(11).}\end{matrix}$

The magnetic field change ΔB_(t) caused by the position change of theultrasonic transducer can be obtained by numerical calculations or byexperimental measurements, which will be described below, respectively.

In a static magnetic field, when the ultrasonic transducer is placed ina water tank, due to the difference in the susceptibility between theultrasonic transducer and the water, the local magnetic field willchange. Accordingly, the magnetic field resulting from the ultrasonictransducer in the water can be obtained through measurements in tests,which can be used as the magnetic field change ΔB_(t) resulting from theposition change of the ultrasonic transducer. In FIG. 3, an MR phaseimage is acquired respectively when the ultrasonic transducer exists inthe water and when the ultrasonic transducer does not exist in thewater, thus measuring the change ΔB_(t) in the magnetic field.

A flowchart of measuring the magnetic field caused by the ultrasonictransducer will be described below with reference to FIG. 3, and theflowchart mainly comprises the following steps:

Step 01, place a water tank 200 in the magnetic resonance equipment,withthe water tank 200 being preferably made of a non-magnetic material,such as plastic, etc.

Step 02, place the ultrasonic transducer 210 in the water tank, and makethe position and direction of the ultrasonic transducer 210 identical tothe position and direction in the practical heating.

Step 03, charge water into the water tank 200.

Step 04, use the magnetic resonance imaging equipment, and use gradientecho sequences to acquire a first phase image φ₁. For example, theparameters of the sequences can be set as T_(R)/T_(E)=20 ms/15 ms.

Record the liquid level in the water tank 200 in step 03 or step 04.

Step 05, take the ultrasonic transducer 210 out of the water tank, andcharge water into the water tank 200 to restore the liquid level at thetime when the ultrasonic transducer 200 was placed therein. During thisperiod of time, the position of the water tank 200 is kept unchanged.

Step 06, use the magnetic resonance imaging equipment to acquire asecond phase image φ₂, wherein the gradient echo sequences use the sameparameters as those in acquiring the first phase image φ₁.

Step 07, obtain the magnetic field ΔB_(t) resulting from the, ultrasonictransducer 210 by calculating according to the first phase image φ₁ andthe second phase image φ₂, for example, the following equation can beused:ΔB _(t)=(φ₁−φ₂)/(γ·T _(E))

During the above process, the support of the ultrasonic transducer 210can be put into and taken out of the water tank together with theultrasonic transducer. This can also compensate at the same time for thetemperature errors caused by the support.

If the geometric structure of the ultrasonic transducer and thesusceptibility of the material can be obtained, then the magnetic fieldresulting from the ultrasonic transducer can be obtained by calculatingwith the numerical calculation method, which can be used as the magneticfield change caused by the position change of the ultrasonic transducer.The method of obtaining ΔB_(t) by numerical calculation will bedescribed below. The method mainly includes the following steps:

Step 11, divide the ultrasonic transducer into a plurality of finitevolume elements.

Step 12, calculate the magnetic dipole moment of each finite volumeelement.

Step 13, calculate the magnetic field produced by the magnetic dipolemoment in each finite volume element at every point in space. Forexample, the magnetic field produced by the magnetic dipole moment ofeach finite volume element at every point in space can be calculated byusing the Biot-Savart-Laplace Law.

Step 14, the magnetic field caused by the ultrasonic transducer at everypoint in space equals the sum of the magnetic fields produced by eachfinite volume element at the point, so by summing the magnetic fieldproduced by each finite volume element the magnetic field ΔB_(t) causedby the ultrasonic transducer can be obtained.

After the magnetic field caused by the ultrasonic transducer has beenobtained, the above equation (10) is used to compensate the temperaturechange in the heated area, thus obtaining the accurate temperaturechange.

In summary, the method in the embodiment of the present inventionproceeds as follows: acquiring an MR phase image as a reference imagebefore the HIFU equipment having heated the area to be heated; acquiringanother MR phase image, as a heated image, when the HIFU equipment isheating or after it has heated. The temperature change of the heatedarea is calculated according to the heated image and the referenceimage. The temperature change is compensated according to the magneticfield caused by the ultrasonic transducer.

Although modifications and changes may be suggested by those skilled inthe art, it is the intention of the inventors to embody within thepatent warranted hereon all changes and modifications as reasonably andproperly come within the scope of their contribution to the art.

1. A method for reducing errors in a measurement of temperature bymagnetic resonance in a high intensity focused ultrasound (HIFU)procedure using magnetic resonance imaging-guide HIFU equipment,comprising: before heating, with said HIFU equipment, an area of asubject located within a magnetic resonance imaging apparatus, operatingsaid magnetic resonance imaging apparatus to acquire a magneticresonance phase image, as a reference image, of a region of the subjectcomprising said area, the operation of said magnetic resonance imagingapparatus including generating a static magnetic field therein, and saidHIFU equipment comprising an ultrasonic transducer comprised of amaterial that causes a change in said static magnetic field when aposition of said ultrasonic transducer changes within said magneticresonance imaging apparatus; operating said magnetic resonance imagingapparatus to acquire another magnetic resonance phase image of saidregion of said patient, as a heated image, during or after heating ofsaid area by said HIFU equipment; supplying said reference image andsaid heated image to a processor and, in said processor, automaticallycalculating a temperature change in the heated area from said heatedimage and said reference image; and in said processor, automaticallymaking compensation to said temperature change by identifying and usingsaid change in said static magnetic caused by a position change of saidultrasonic transducer in said magnetic resonance apparatus.
 2. Themethod as claimed in claim 1, comprising making the compensation to saidtemperature change according to the equation,${\Delta\; T} = {{\Delta\; T_{conv}} - \frac{\lbrack {{\Delta\; B_{t{({r\; 2})}}} - {\Delta\; B_{t{({r\; 1})}}}} \rbrack \cdot}{B_{0} \cdot \alpha \cdot}}$wherein, ΔT represents a value of the temperature change after beingcompensated; ΔT_(conv) represents the value of the temperature changecalculated according to said heated image and said reference image;[ΔB_(t)(r2)−ΔB_(t(r1))] represents the change in the magnetic fieldcaused by the position change of said ultrasonic transducer from aposition r1 to position r2; B₀ represents an intensity of the staticmagnetic field intensity; and α represents a temperature frequencycoefficient.
 3. The method as claimed in claim 1, comprising measuring amagnetic field produced by said ultrasonic transducer in water, andusing said change in the static magnetic field caused by the positionchange of said ultrasonic transducer.
 4. The method as claimed in claim3, wherein said ultrasonic transducer has a support, and comprising,during the measuring of the magnetic field caused by said ultrasonictransducer, putting said support into and taking said support out ofwater together with said ultrasonic transducer.
 5. The method as claimedin claim 3, comprising using the equationΔB _(t)=(φ₁−φ₂)/(γ·T _(E)) to calculate the magnetic field ΔB_(t) causedby said ultrasonic transducer, wherein φ₁ represents a first phase imageacquired when the ultrasonic transducer is in water; φ₂ represents asecond phase image acquired when the ultrasonic transducer is not inwater; γ represents the gyromagnetic ratio of hydrogen atomic nuclei;and T_(E) represents an echo time.
 6. The method as claimed in claim 1,comprising calculating a magnetic field caused by said ultrasonictransducer as the change of the static magnetic field caused by theposition change of said ultrasonic transducer.
 7. The method as claimedin claim 6, comprising calculating the magnetic field caused by saidultrasonic transducer by: dividing said ultrasonic transducer into aplurality of finite transducer volume elements; calculating a magneticdipole moment of each finite transducer volume element, and calculatinga magnetic field in space produced by the magnetic dipole moment of eachfinite transducer volume element; and summing the magnetic field inspace produced by each finite volume element to obtain the magneticfield caused by said ultrasonic transducer.